Brain injuries, including traumatic brain injury (TBI) and stroke, affect well over 2 million Americans each year and are a significant health concern worldwide. There are currently approximately 5.7 million stroke survivors in the US, many with permanent disabilities, and more than 5 million Americans who have suffered a TBI that in some cases results in the permanent need for help in performing daily activities. Traumatic brain injuries result from a blow or jolt to the head or a penetrating head injury that disrupts the function of the brain, with severity ranging from “mild,” i.e., a brief change in mental status or consciousness to “severe,” i.e., an extended period of unconsciousness or amnesia after the injury. In contrast, strokes are a result of diseases that affect the blood vessels that supply blood to the brain. A stroke occurs when a blood vessel that brings oxygen and nutrients to the brain either bursts (hemorrhagic stroke) or is clogged by a blood clot or some other mass (ischemic stroke). The majority of strokes are ischemic, however hemorrhagic strokes typically result in more severe injuries.
Despite several decades of effort, scientists have not yet found a pharmacological agent that consistently improves outcomes after stroke or TBI (see Sauerland, S. et al., Lancet 2004, 364, 1291-1292; Brain Trauma Foundation, American Association of Neurological Surgeons, Joint Section on Neurotrauma and Critical Care. Guidelines for the management of severe head injury. J. Neurotrauma 1996, 13, 641-734).
After TBI or stroke, inflammation is a principle cause of secondary damage and long-term damage. Following insults to the central nervous system, a cascade of physiological events leads to neuronal loss including, for example, an inflammatory immune response and excitotoxicity resulting from disrupting the glutamate, acetylcholine, cholinergic, GABAA, and NMDA receptor systems. In these cases, a complex cascade of events leads to the delivery of blood-borne leucocytes to sites of injury to kill potential pathogens and promote tissue repair. However, the powerful inflammatory response has the capacity to cause damage to normal tissue, and dysregulation of the innate, or acquired immune response is involved in different pathologies.
In addition to TBI and stroke, inflammation is being recognized as a key component of a variety of nervous system disorders. It has long been known that certain diseases such as multiple sclerosis result from inflammation in the central nervous system, but it is only in recent years that it has been suggested that inflammation may significantly contribute to neurodegenerative disorders such as HIV-related dementia, Alzheimer's and prion diseases. It is now known that the resident macrophages of the central nervous system (CNS), the microglia, when activated may secrete molecules that cause neuronal dysfunction, or degeneration.
There is growing experimental evidence that progesterone, its metabolites and other gonadal steroids such as estrogen and possibly testosterone, are effective neuroprotective agents. Pre-clinical and clinical research demonstrates that the hormone progesterone is a potent neurosteroid that, acutely administered, can dramatically reduce cerebral edema, inflammation, tissue necrosis, and programmed cell death (see Djebaili, M. et al, J. Neurotrauma 2005, 22, 106-118; Pettus, E. H. et al., Brain Res. 2005, 1049, 112-119; Grossman, K. J. et al., Brain Res, 2004, 1008, 29-39; He, J. et al., Exp. Neurol. 2004, 189, 404-412; He, J. et al., Restor. Neurol. Neurosci. 2004, 22, 19-31; Djebaili, M. et al., J. Neuroscience 2004, 123, 349-359; Hoffman, S. W. et al., Academy of Emergency Medicine, 2001, 8, 496-497; and Wright, D. W. et al., J. Neurotrauma. 2001, 18, 901-909).
In vivo data has demonstrated progesterone's neuroprotective effects in injured nervous systems. For example, following a contusion injury, progesterone reduces the severity of post injury cerebral edema. The attenuation of edema by progesterone is accompanied by the sparing of neurons from secondary neuronal death and improvements in cognitive outcome (Roof et al. (1994) Experimental Neurology 129:64-69). Furthermore, following ischemic injury in rats, progesterone has been shown to reduce cell damage and neurological deficit (Jiang et al. (1996) Brain Research 735:101-107). A Phase II, single-center, controlled trial involving 100 moderate to severe TBI patients showed that 3 days of intravenous progesterone treatment reduced mortality by over 60% and significantly improved functional outcomes at 30 days post-injury (see Wright, D. A. et al., Ann. Emerg. Med. 2007, 49, 391).
PCT Publication WO 02/30409 to Emory University provides methods for conferring a neuroprotective effect on a population of cells in a subject following a traumatic injury to the central nervous system by administration of a progestin or progestin metabolite following a traumatic brain injury.
PCT Publication WO 06/102644 also to Emory University provides methods for the treatment or the prevention of neuronal damage in the CNS by tapered administration of a progestin or progestin metabolite following a traumatic or ischemic injury to the CNS to avoid withdrawal.
PCT Publication No. WO/2006/102596 to Emory University provides certain methods of treating a subject with a traumatic central nervous system injury, more particularly, a traumatic brain injury that include a therapy comprising a constant or a two-level dosing regime of progesterone.
Although progesterone has been shown to be neuroprotective in traumatic brain injury, its efficacy in stroke is less well defined. However, studies have indicated that progesterone may be useful in treating or preventing neurodegeneration following stroke (see Stein, D. (2005) The Case for Progesterone US Ann. N.Y. Acad. Sci. 1052:152-169; Murphy, et al. (2002) Progesterone Administration During Reperfusion, But Not Preischemia Alone, Reduces Injury in Ovariectomized Rats. J. Cereb. Blood Flow & Metab. 22:1181-1188; Murphy, et al. (2000) Progesterone Exacerbates Striatal Stroke Injury in Progesterone-Deficient Female Animals. Stroke 31:1173).
U.S. Pat. No. 6,245,757, now expired, to Research Corporation Technologies, Inc. provides a method for the treatment of ischemic damage, such as damage due to stroke or myocardial infarction comprising administering to a mammal afflicted with stroke an effective amount of a neuroprotective steroid in a suitable vehicle.
In addition to being a gonadal steroid, progesterone also belongs to a family of autocrine/paracrine hormones called neurosteroids. Neurosteroids are steroids that accumulate in the brain independently of endocrine sources and which can be synthesized from sterol precursors in nervous cells. These neurosteroids can potentiate GABA transmission, modulate the effects of glutamate, enhance the production of myclin, and prevent release of free radicals from activated microglia.
Various metabolites of progesterone have also been suggested to have neuroprotective properties. For instance, the progesterone metabolites allopregnanolone or epipregnanolone are positive modulators of the GABA receptor, increasing the effects of GABA in a manner that is independent of the benzodiazepines (Baulieu, E. E. (1992) Adv. Biochem. Psychopharmacol. 47:1-16; Robel et al. (1995) Crit. Rev. Neurobiol. 9:383-94; Lambert et al. (1995) Trends Pharmacol. Sci. 16:295-303; Baulieu, E. E. (1997) Recent Prog. Horm. Res. 52:1-32; Reddy et al. (1996) Psychopharmacology 128:280-92). In addition, these neurosteroids act as antagonists at the sigma receptor, which can activate the NMDA channel complex (Maurice et al. (1998) Neuroscience 83:413-28; Maurice et al. (1996) J. Neurosci. Res. 46:734-43; Reddy et al. (1998) Neuroreport 9:3069-73). These neurosteroids have also been shown to reduce the stimulation of cholinergic neurons and the subsequent release of acetylcholine by excitability. Numerous studies have shown that the cholinergic neurons of the basal forebrain are sensitive to injury and that excessive release of acetylcholine can be more excitotoxic than glutamate (Lyeth et al. (1992) J. Neurotrauma 9(2):S463-74; Hayes et al. (1992)J. Neurotrauma 9(1):S173-87).
As discussed above, following a traumatic injury to the central nervous system, a cascade of physiological events leads to neuronal loss. In addition, the injury is frequently followed by brain and/or spinal cord edema that enhances the cascade of injury and leads to further secondary cell death and increased patient mortality. Methods are needed for the in vivo treatment of traumatic CNS injuries that are successful at providing subsequent trophic support to remaining central nervous system tissue, and thus enhancing functional repair and recovery, under the complex physiological cascade of events which follow the initial insult.
Although successful, the use of progesterone itself as a therapeutic suffers from several drawbacks, including most notably its poor solubility and its potential for generating unwanted side effects. Therefore, there is a need for progesterone analogues that exhibit improved solubility and bioavailability for the treatment of ischemic, hemorrhagic or traumatic central nervous injury.